1. Field of the Invention
The present invention relates to a semiconductor laser and, more particularly, to a semiconductor laser array which includes a plurality of stripe-shaped stimulated regions. The plurality of stripe-shaped stimulated regions are phase-coupled to each other so that the laser beams are coupled to each other with a phase difference of zero degrees, thereby obtaining a high power laser beam.
2. Description of the Related Art
A semiconductor laser has widely been used as a light source in an optical information processing system such as an optical communication system and a digital audio disc system. Further, the semiconductor laser is widely used in an optical disc system wherein new information can be written into the optical memory disc through the use of a semiconductor laser, the output power of which is modulated in accordance with the information to be written into the optical memory disc. Rapid processing is required in such an optical information processing system as the amount of information to be handled increases. To ensure the rapid processing, the semiconductor laser must emit the laser beam at a high power level in a stable operating range. However, in the conventional semiconductor laser having a single stimulated region, the practical maximum output is about 40 mW.
To enhance the output level, a semiconductor laser array has been proposed, wherein a plurality of stimulated regions are aligned in a parallel fashion, and the plurality of stimulated regions are optically, phase coupled to each other so as to emit the laser beam in a single phase. This is referred to as a phase coupled laser array. The phase coupled semiconductor laser array is effective to converge the laser beams in a narrow radiation angle.
In the conventional semiconductor laser array of the gain guide type, the gain is substantially reduced at the coupling region positioned between two adjacent laser emitting regions and, therefore, the electric field has a phase difference of 180 degrees at two adjacent laser emitting regions. The far field pattern has a plurality of peaks as shown in FIG. 8. Thus, the conventional semiconductor laser array of the gain guide type cannot be of practical use.
To improve the above-mentioned problems, a semiconductor laser array of the index guide type has been proposed. For example, D. E. Ackley et al of Hewlett-Packard Laboratories proposed a semiconductor laser array of a buried heterostructure laser with a leaky mode (Appl. Phys. Letters, vol. 39, 1981, pp. 27). The proposed laser array ensures an effective coupling of the laser emitting regions, but has two peaks in the far field pattern because of the leaky mode.
D. Botez et al of RCA Laboratories proposed a CSP-LOC (Channeled-Substrate-Large-Optical-Capacity) laser (document of IOOC, 1983, 29B5-2). The proposed semiconductor laser utilizes the distribution of effective refractive index which is formed by a coupling to the GaAs substrate. The region disposed between two adjacent laser emitting regions has a high absorption coefficient. The refractive index difference is not obtained when the absorption coefficient is minimized. Accordingly, it is difficult to reduce the phase difference between two adjacent laser emitting regions to zero.
D. E. Ackley et al of Hewlett-Packard Laboratories further proposed the semiconductor laser array of the ridge-type (Appl. Phys. Letters, vol. 42, 1983, pp. 152). Each pair of adjacent laser emitting regions has a phase difference of 180 degrees because of the high absorption caused by an electrode disposed at a coupling region of the adjacent laser emitting regions.